It is important to know the nature and character of soil at selected locations in the vadose or unsaturated zone to determine if it may contain toxic materials or may be contaminated in some way which would make it suitable or unsuitable for specific purposes.
Water, which is contained within the soil, is representative of the soil in many respects, and the art has sought effective ways to collect the soil water at selected locations so that the water obtained may be analyzed, its contents identified, and their concentrations determined.
A method used in early attempts to recover soil water involved taking a sample of the soil and putting it under pressure to mechanically squeeze out the water. Devices were later developed called lysimeters which contain a sampling chamber and which use a vacuum to draw moisture from the soil through a filter into the sampling chamber. These devices have their limitations and they introduce inaccuracies in the collection of the water so that the soil moisture recovered has not been truly representative of the soil moisture under test. In an effort to reduce these inaccuracies there were attempts to use polyethylene porous tubes in the lysimeter. Then the focus of the art was turned to the use of porous ceramic filters, and attempts were made to preclean and treat these ceramic filters by leaching the porous ceramic material with hydrochloric acid followed by rinsing with deionized water. The porous ceramic filters were found to absorb or to interfere with the chemical constituents NO.sub.4, NO.sub.3, NO.sub.2, PO.sub.4, P and S especially when the sorptive capacity of the soil is less than that of the ceramic material. The porous ceramic filters were found to attenuate concentrations of Ni, Cu, Pb, Z.sub.n, Fe and Mg, and were found to be unsuitable for fecal coliform analysis.
When using porous ceramic materials in the lysimeter, the porous ceramic material usually has been attached to a polyvinylchloride (PVC) tubing by use of an adhesive, but it has been found that both the PVC tubing and the adhesive used to secure it contribute to inaccuracies of the collected sample. Further, the ceramics are extremely fragile and it is difficult to assemble the ceramic filters in lysimeters, to install the lysimeters in a selected location and to remove the soil water collected without fracturing or breaking the ceramic materials.
An object of the present invention is to discover how to make an improved filter material which is structurally strong and chemically inert which can be used in a lysimeter through which moisture from the soil may be passed and then recovered from the lysimeter for testing, without disturbing the characteristics and contents of the soil moisture and without contributing elements to or subtracting elements from the soil water due to the material itself, and which can be used for collecting soil water which is truly representative of the water contained in the soil being tested.
Although there have been many known substances which may be considered to be inert, to my knowledge none of these have characteristics of moisture transmission which would make them suitable for use in soil water recovery.
There is a type of plastic resins which are basically monomers containing one or more atoms of fluorine or copolymers of such monomers with other monomers, the fluorine-containing monomers being the greater part of the mass. These plastics are called fluoroplastics and have been used as coatings, linings and as components of pumps, fittings, process vessels, etc. Fluoroplastics include polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), ethylenechlorofluoroethylene copolymer (ECTFE), polyvinylidene fluoride (PVDF), ethylenetetrafluoroethylene copolymer (ETFE), perfluoroalkosy resin (PFA), and fluorinated ethylene-propylene copolymer (FEP). As now known and used, these resins are neither rigid nor porous. They present a tight barrier to transmission of either liquids or gases. They would clearly be unsuitable for any filtering function.
I have discovered that it is possible to use these fluoroplastic resins as raw materials in a process for preparing synthetic resins which are rigid and porous, which can be molded and machined as filter units in a lysimeter and that a lysimeter which employs such special filter units is much more effective in the collection of soil water which is truly representative in content and character of the moisture contained in the soil at the location selected for test.